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We'd had our new kittens for about two weeks when one of them developed diarrhea. It was a stinky mess in the litterbox -- and one that had me worried something was seriously wrong. A quick consultation with a veterinarian helped us figure out that the culprit was the leftover milk the kitten had lapped out of a cereal bowl one morning. Though I had shooed her away from it, she drank just enough to upset her tender tummy.

Like human babies, animal babies are susceptible to all sorts of illnesses and conditions. Here's what you should know.

Puppies and Kittens: Why You Need a Well Visit

Your first step after getting your pet is to take her to a veterinarian to make sure she's gotten the vaccinations she needs. The vet will also give her a thorough physical exam, including checking her stool for signs of internal parasites (not unusual in young animals).

"This first visit is important for all new pets," says Scott Shaw, DVM, an assistant professor of clinical sciences at the Cummings School of Veterinary Medicine at Tufts University. But it's especially important if you get them from sources other than adoption agencies or breeders, he notes. "When you get animals as strays or through other families, you never know what health issues might be smoldering."

Kittens and puppies are vulnerable to internal parasites (such as roundworms, hookworms, and coccidiosis) and external parasites (such as ear mites, fleas, and ticks). Symptoms of internal parasites include diarrhea, a potbelly, and weight loss. Symptoms of external parasites include scratching, dandruff, and black crust inside the ears.

Medical Problems in Kittens

Upper respiratory infections. Kittens pick up respiratory viruses fairly easily, and they often develop secondary bacterial infections as well. Symptoms include sneezing, eye discharge, and loss of appetite. "Because they're so young, they just become miserable little things," Shaw says.

Accidents. "We see a fair number of kittens who get sat on or crushed in reclining chairs," Shaw says. "Owners need to be very, very careful. "Kittens can also sprain or fracture their legs if they fall from counters or table tops and tear their nails if they get caught in fabric."

Medical Problems in Puppies

Pneumonia. This condition is so common that "it's unusual for a day to go by that we don't have a puppy in the intensive care unit getting treated," Shaw says. "It's most often caused by bacteria, but the stress of traveling to a pet store or new home also plays a role." This is a life-threatening disease that requires veterinary care.

GI issues. Puppies' gastrointestinal systems are immature and therefore vulnerable to stress (like going to a new home). Symptoms include vomiting, diarrhea, lack of appetite, and bloating.